1. Field of the Invention
This invention relates to electronic servo circuits for controlling electric motors.
2. Description of the Prior Art
Present-day electric apparatus often utilizes direct current motors to rotate or move mechanical parts. The rotational speed and power output of such motors are generally controlled by means of servo circuitry which controls the electric power applied to the motor. The servo circuitry receives as inputs various measured parameters, such as position of the motor shaft or the torque produced by the motor, processes these inputs, and produces an output signal to control the motor.
Often, it is desirable to maintain a constant motor speed by means of a servo circuit. In such a case, the motor shaft is generally connected to a tachometer which produces a varying output, often a train of pulses, with a frequency related to the rotational speed of the motor. A frequency-to-voltage converter is then used to convert the output developed by the tachometer into a DC voltage which then can be processed by standard amplifier circuitry into the power output necessary to control the motor.
Many prior art circuits have been designed to perform the necessary frequency-to-voltage conversion. In one such system, each output pulse produced by the tachometer is applied to a monostable multivibrator which, after a predetermined time interval, sets a latch. The latch is reset by the next tachometer pulse. The latch circuit produces a square wave output whose duty cycle is proportional to the error between the actual motor speed and the desired motor speed. The output of the latch is connected in a feedback circuit so that the output duty cycle and, thus, the error is minimized during circuit operation.
Another prior art circuit utilizes phase comparison to control motor speed. In this type of circuitry the output pulses produced by the tachometer are applied to one input of a phase comparator. A reference oscillator is connected to the other input of the phase comparator. The phase comparator produces an output which is proportional to the difference in phase between the tachometer output and the reference signal; this output is amplified and used to drive the motor.
Although the above prior art circuits perform the required control function, each has its own problems. The multivibrator circuit described above operates to adjust motor speed to a constant determined by the time constant of the multivibrator. This time constant is in turn determined by values of electronic components which are subject to change by thermal effects and aging. Thus to insure constant speed the components used in the multivibrator must be (1) precision components, (2) temperature compensated, or (3) adjusted at the time of manufacture; each alternative is expensive. A second problem with the multivibrator circuit is that it needs additional circuitry to start the motor from a power-off condition, since the servo loop may not be self-starting or may be slow in starting.
The phase comparison circuitry requires a phase comparator circuit which is generally complicated and expensive.